The present invention relates to a press section of a paper machine for removal of water from a web, such as a paper web.
Such a press section is known from European Patent. 0 107 606 B1. In accordance with FIG. 1 of said patent, two pairs of press rolls are provided, each forming a press nip. An endless felt belt which removes water from the web, hereinafter referred to as the "water removal felt", travels through the nip. Furthermore, an endless elastic press element is present in the form of a belt which travels in succession through the two nips and over several guide rolls. The web from which the water is to be removed comes in contact with the elastic belt in the first nip and is then conducted by the belt into the second nip. By this arrangement, it is intended to achieve the result that the two water removal felts move away from the web from which the water is to be removed directly behind the two nips. Therefore, the transport of the belt behind the two nips takes place completely without participation of the two water removal felts. In this way, rewetting of the web from the water removal felts is avoided. In this way, it is attempted to increase the water removal capacity of the press section and, therefore, to obtain an increased solids content of the web when it leaves the press section.
One disadvantage of this known press section is, however, that the same side of the web is in contact with the elastic belt in both nips. Accordingly, the other side of the web is in contact with the water removal felt in both press nips. The result of this is that the final web (for instance, web of paper) has different properties on the two sides, for instance, different smoothness or absorbability.
In the development of future paper machines, it is desirable that the press section meet the following requirements:
1. The number of press nips should be as small as possible and, insofar as possible, should not exceed two in number. PA1 2. Within each press nip the web should travel between a water removal felt and an elastic press element. PA1 3. The use of granite or similar stone or synthetic-stone rolls should be avoided. In many cases, the use of suction press rolls should also be avoided. PA1 4. Upon passage through the press section, the web should be in continuous contact with a transport means, for instance with a felt belt or with an elastic press element (i.e. closed path of the web without free web section). PA1 5. The finished product as far as possible should have identical properties on its two sides. PA1 6. The water removal capacity of the press section should be increased as much as possible over what it has been up to now; i.e. the highest possible solids content of the web when it leaves the press section should be obtained despite a very high operating speed (of the order of magnitude of 2,000 meters per minute or more). PA1 a) either as a press belt which travels within the press nip around a press roll or over a press shoe and, furthermore, over guide rolls, PA1 b) or as a liquid-tight press shell closed at both ends, of a shoe-press unit, PA1 c) or as a liquid-tight press shell closed at both ends which travels loose over a roll body, PA1 d) or as an elastic covering of a press roll.